MHRA's experts found that soft tissue necrosis may occur even in patients who were not showing any symptoms such as hip pain or swelling; therefore it issued the new guidance, aimed at doctors.

The Medicines and Healthcare products Regulatory Agency (MHRA) has issued an alert for patients with "metal-on-metal" hip implants – believing that they would be at risk of bone or muscle damage from the devices.

Approximately 56,000 patients in the UK have such devices implanted in them and will be recalled for a series of tests, including MRI scans and blood tests, due to concerns over toxicity.

A large number of these patients already attend annual checkups due to previous concerns over the devices. The checkups include questionnaires, blood metal tests and scans to determine if metal particles have eroded off the implants and deposited in surrounding tissue. If this occurs, revision surgery may be needed.

The safety and durability concerns about metal-on-metal hip replacements and a scandal involving faulty breast implants, were few of the main reasons in the EU's decision to overhaul its medical device and in vitro diagnostics regulations.

Previously, patients with smaller sizes of devices were considered low risk by the MHRA, apart from certain implants made by Johnson & Johnson's subsidiary DePuy Synthes, which were thought to be riskier than other type of metal-on-metal hip implants.

In 2010, DePuy recalled two models of its ASR hip replacement systems and in 2013, the UK's NHS halted most metal-on-metal hip replacements due to high failure rates. Last year, J&J and DePuy were ordered to provide compensation of slightly more than USD1 billion to six patients who were fitted with J&J's Pinnacle hip implants.

However, the MHRA has recently suggested that all patients with "metal-on-metal" hips ̶ including those who have not encountered problems so far ̶ should now be followed up.

This means thousands of extra patients will need to undergo testing, although the MHRA said it would not necessarily mean an increase in the number of revision surgeries.

All metal implants considered a risk now

"We've now included a group of people who were at low risk, but were not at no risk. It's more about catching people who could be developing these complications early," said Dr Neil McGuire, the MHRA's clinical director of medical devices.

MHRA's experts found that soft tissue necrosis may occur even in patients who were not showing any symptoms such as hip pain or swelling; therefore it issued the new guidance, aimed at doctors.

"We have updated current advice to endure patients with metal-on-metal hip implants continue to receive appropriate follow up to detect emerging complications should they arise," said McGuire.

"The clinical advice we have received indicates patients will likely have the best outcomes if these problems are detected early, monitored and treated if necessary," he added. MIMS